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| Titel |
Late Quaternary landscape evolution in the Great Karoo, South Africa: Processes and drivers. |
| VerfasserIn |
Chris Oldknow, Janet Hooke, Andreas Lang |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250122059
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| Publikation (Nr.) |
 EGU/EGU2016-990.pdf |
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| Zusammenfassung |
| The Great Karoo spans the north-central part of South Africa at a major climatic boundary.
The characteristics, sequences, spatial patterns and drivers of river response to Late
Quaternary climate changes in this region remain unclear due to the fragmentary
alluvial/colluvial stratigraphic record and the lack of dated palaeoclimatic archives. Dendritic
gully networks incised into deep deposits (up to 6 m) of colluvium and alluvium in the upper
Sundays River catchment expose a legacy of “cut and fill” features. In 1st order
tributaries, these are predominantly discontinuous palaeochannels and flood-outs with
localised palaeosols, whereas in 2nd & 3rd order tributaries there are: 1) incised
palaeo-geomorphic surfaces, 2) semi-continuous inset terrace sequences, 3) buried
palaeo-gully topography. Using a combination of field mapping, logging of sediment
outcrops, soil micromorphological and grain size analysis, mineral magnetic measurements
and radiometric dating (OSL & 14C), we derive a stratigraphic evolution model which
demonstrates a) the number of phases of incision, aggradation and pedogenesis, b) the spatial
and temporal extent of each phase and c) the drivers of alluviation and associated
feedbacks.
Our reconstruction of regional valley alluviation indicates four distinct terrace units of
contrasting depositional age. The base of the succession reflects slow aggradation under
periglacial conditions associated with the Last Glacial Maximum. Subsequent channel
entrenchment, causing terrace abandonment (T1) occurred in the deglacial period when
vegetation and rainfall were in anti-phase. Re-instatement of connectivity with deep upland
colluvial stores resulted in the injection of a pulse of sediment to valley floors, triggering
compartmentalised backfilling (aggradation of T2) which propagated upstream as far as the
second order drainage lines. This backfilling restructured the local hydrology, which, in
concert with enhanced summer-rainfall, contributed to a major increase in the palaeo-water
table, enhanced vegetation productivity and led to the formation of extensive calcified
root-mats. Soil micromorphological evidence from this calcrete unit and burial of T2 by up to
1.5 m of alluvium (T3) indicates subsequent aridification, but depth of channel entrenchment
was retarded by the blanketing effect of the underlying calcrete. The final terrace (T4) is
much younger (Late Holocene), reflecting slow aggradation in a wetland setting.
Wider segments of valley preserve a ‘cut and fill’ phase intermediate in age between
regional T3 and T4 which appears to be a function of varying alluvial preservation
potential. The research demonstrates that phases of alluviation and pedogenesis in these
valleys reflect a complex interplay between Late Quaternary climate change and
autogenic-feedbacks relating to abrupt changes in sediment supply and connectivity. |
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